1. Field of the Invention
The present invention relates to a thermal fixing device and an image forming apparatus including the thermal fixing device.
2. Description of the Related Art
An image forming apparatus such as a laser printer is generally provided with a thermal fixing device including a heat roller and a pressing roller, and a toner transferred onto a sheet is thermally fixed during a period when the sheet passes through between the heat roller and the pressing roller.
In such a thermal fixing device, there is known one in which in order to increase a contact area between a heat roller and a sheet and to achieve quick and certain fixation, plural pressing rollers are provided in a conveyance direction of the sheet. However, when the plural pressing rollers are provided, there is a case where an image shift occurs when a sheet having passed through a nip portion between a first pressing roller at an upstream side in a conveyance direction of the sheet and a fixing member enters a nip portion between a second pressing member at a downstream side and the fixing member.
Thus, for example, JP-A-5-273881 discloses that a pressing force of a first pressing roller is made larger than a pressing, force of a second pressing roller, and a nip amount between the first pressing roller and a fixing roller is made larger than a nip amount between the second pressing roller and the fixing roller, so that fixing properties are improved.